The present invention relates to motor drives and in particular to a ground fault detection system identifying locations of ground faults in motor drive systems.
Motor drive systems provide control of power applied to a motor for improved motor control and efficiency. Generally, such systems receive a source of alternating current (AC) which is rectified to provide a direct current (DC) bus. The DC voltage of the bus is then received by an inverter which synthesizes new AC waveforms to be applied to the motor. The new waveforms, for example, may provide a variable control frequency for regulating the speed of the motor or for controlling its torque or dynamic characteristics such as startup and slowdown.
The circuitry of the motor drives and the windings of motors attached to the motor drives are typically each enclosed in grounded metal housings. Should a powered conductor of either break free and contact the housing, the power of the conductor is safely shunted to ground. Such failures are called “ground faults”.
Ground faults in high-powered equipment can produce high current flow into ground of a magnitude that can damage the equipment or present other hazards. This ground fault current can be limited by using high resistance ground connections in which a series resistor is placed in the ground path to limit current flow. Such high resistance grounding can in many cases permit continued operation of the equipment during a ground fault. By the same token high resistance grounding can make it difficult to determine that a ground fault has occurred and/or to pinpoint the particular device having a ground fault. Even when the particular motor drive affected is identified, it may be difficult or impossible to determine the location of the ground fault and the appropriate response.